1. Field of the Invention
This invention relates to a hard aluminum plating film produced by an aluminum electroplating method, and a production method for the same.
2. Description of the Related Art
An aluminum electroplating method is known as one of the plating methods which can respond to a rise of environmental awareness, since heavy metals which affect the environment and a human body are included in neither a plating bath nor a plating film. The surface of the coated layer made of aluminum can be excellent in abrasion resistance, corrosion resistance, coloring, etc. by anodizing it, therefore, many studies are done for practical use. As a method of forming the aluminum plating film, a hot-dip method is generally known. In an aluminum hot-dip coating method, aluminum is molten at high temperature such as around 700° C., and a sample is immersed in it. Therefore, the coating can be made easily, however, on the other hand, it is not easy to control the film thickness, and pinholes or voids can easily occur. Furthermore, the melting point of a sample made of magnesium is 648.8° C. for example, therefore, in the aluminum melt, since the sample is molten the coating cannot be made.
On the other hand, film thickness can be controlled by adjusting the quantity of electric charge in the aluminum electroplating method, and the aluminum can be plated at a low temperature of 200° C. or lower. The standard electrode potential in aluminum plating is −1.71V, and is baser than the electrode potential in hydrogen generation, such that plating aluminum from solution is very difficult. Therefore, aluminum electroplating can be done using a nonaqueous solvent or fused salt.
As the organic nonaqueous solvent, a plating bath using toluene solvent was reported in 1956 by Ziegler and Lehmkuhl, and is widely used known as the Sigal Process (tradename). This plating bath is composed mainly of triethyl aluminum, sodium fluoride, and toluene, and triethyl aluminum used as solute has strong water-reactivity and strong spontaneous combustibility, therefore, the plating machine must be explosion proof. Also, cleaning process after the plating, or wasting the cleaning solution, is not easy, either, and study on these processes is needed.
As one of other nonaqueous solvents for plating baths, plating bath using aluminum chloride, lithium hydride aluminum, and diethylether (Hydride type plating bath) was reported in 1952 by Brenner et. al, and it is known well. Also, a plating bath using tetrahydrofuran solvent in which liquid stability has been improved, is reported and widely used. However, lithium hydride aluminum or lithium hydride used as an additive, is very active chemically, and is not used currently because of potential for explosions.
On the other hand, plating methods for various alloy systems using fused salt are reported, however, no plating method which can be used practically is found, except for a plating method for manganese-aluminum alloy. The plating bath used here is mainly composed of aluminum chloride, sodium chloride, and potassium chloride, and a small amount of manganese chlorides are added. In this fused salt plating method, because reagent containing chlorides mainly is used, the plating machine utilized corrodes after using it for a long time. There is also a report that after melting alkylpyridinium halide, quaternary ammonium halide, alkylimidazolium halide, onium halide, and aluminum halide at the temperature of around 100° C., and plating electrically, plating film is obtained (patent document 1: JP, 2755111,B). However, the reagents are very expensive, and the initial making up process of the bath becomes complicated, therefore, it is unsuitable for practical use.
Here, in patent documents 2 (JP,2004-76031,A (a claim, FIG. 1, FIG. 2)) and 3 (JP,2006-161154,A), it is shown that aluminum can be electroplated by a plating bath using dialkylsulfone such as dimethylsulfone. According to patent documents 2 and 3, after mixing the anhydrous salt of the metal which is made to be the plating film in dimethylsulfone, heating this mixture up to about 110° C., melting of the metal anhydrous salts, finally, the plating bath is made up. In the plating bath, the metal complex ion in which dimethylsulfone is coordinated, is generated and after electroplating, the metal contained in the metal complex ion is reduced and precipitated on the surface of a cathode (base material), and the plating film is formed. Since water does not exist in this plating bath, electrolysis of water does not occur, and plating film made of a metal with low reduction potential can be formed, and since potential for explosion of dimethylsulfone during contact with air is low, it is supposed that it can be used very safely. Currently, there is no restriction for environment on the dimethylsulfone currently used for this plating bath, toxicity like other organic solvents is not reported, either. The melting point of dimethylsulfone is 102-109° C., therefore, in fused salt plating methods, there is an advantage that process temperature can be made comparatively low.
However, the aluminum electroplating film obtained by the above-mentioned conventional technology is soft, before anodic oxidation, therefore, uses of the film were restricted, because the film is easily damaged during the handling of the sample. It was shown that functions caused by characteristics of inactive particulates are given to the plating film, by dispersing the inactive particulates uniformly in the plating film in the plating method of patent document 1, however, it is not so easy to disperse these particulates uniformly.
Therefore, the purpose of this invention is, to provide a plating film with enough hardness before anodic oxidation, which is hard to be damaged during handling, and also to provide the production method of the plating film.